The present invention relates to a method for treating spent abrasive slurry obtained from a process for cutting a body of a substrate material into wafer-like slices. Typically, abrasive slurries are applied in the cutting of semiconductor materials, e.g., ingots from single crystals or polycrystalline silicon, GAs and Ge by means of wire saws into semiconductor wafers and comprise a lubricant or cooling fluid of high viscosity such as a mineral oil or water-soluble liquids (e.g., polyethylene glycol) and a particulate matter from abrasives such as silicon carbide.
While the wires used for cutting the ingots into slices have a smooth surface, the cutting effect is obtained by use of the highly viscose abrasive slurry which is fed to the contact area of the cutting wire and the substrate material to be cut into slices.
During the cutting operation, the substrate material is ground at a so-called saw kerf into powdery material. The slurries also serve to remove such powder substrate material from the saw kerf.
During the cutting operation, the abrasive slurries are contaminated in three ways: The substrate material (e.g. silicon or other semiconductor material) is disintegrated into particles that are taken up by the slurry. The metal of the cutting wires (primarily iron) is another source for particulate contaminations due to surface wear of the wire. Finally, the grains of the abrasive material itself are partially disintegrated into smaller particles which are, of course, also incorporated in the abrasive slurry.
As the concentrations of these three contaminants (in the following called fines) increase over time, the efficiency of the cutting operation decreases. When the slurry finally becomes ineffective, i.e., spent or exhausted, it must be discarded.
The spent slurry is either been disposed of (by incineration or other means) or regenerated.
The methods for regenerating spent abrasive slurries which were heretofore proposed rely on two different principles. The first one starts with the separation of the exhausted slurry into a first liquid fraction and a first solids fraction. Afterwards, the two fractions are regenerated separately, involving various operations such as diluting, washing, classification, filtration, etching, evaporation and others. The regenerated liquid and the regenerated solids fraction, the latter having no or a reduced content of fines, can be used for the preparation of fresh slurry. Examples of such processes are disclosed in WO 2002/096611 and in EP 1 561 557 A1.
There are numerous disadvantages to this method, and the most prominent one is its complexity. The large number of operations needed and in addition the high degree of interactions between these operations would make a small system which can be located and operated near the point of use to regenerate the spent slurry from a local user rather expensive. Therefore, this technology has been used in large centralized units which make it necessary that the spent slurry is transported from the facilities of the users to the regeneration site. The transport costs involved in the regeneration of the abrasive slurry are therefore substantial.
A certain improvement might be possible by lowering the viscosity of the spent slurry by heating the same as is proposed in U.S. Pat. No. 6,231,628 B1. However, the heating of rather highly concentrated slurry creates the risk of scaling, encrustation, fowling and abrasion of heat transfer surfaces resulting in lower heat transfer efficiency, limited life time of the heater and increased energy costs.
The other principle used for slurry regeneration is based on the use of a combination of two centrifuges. In the first centrifuge, a classification of particles is achieved, i.e., the slurry is divided into two fractions that contain both a part of the lubricant and a part of the solids. The two fractions differ in their solids concentration and in particle size distribution of the solids. The overflow of the first centrifuge mainly contains the majority of the lubricant and the smaller particles (fines or debris from semiconductor material, wire and abrasive material, typically below about 10 μm). The sludge resulting from the first centrifugation step (flowable concentrate) contains preferably the abrasive grains with sizes near to those of new slurry (essentially above about 10 μm). The volume of the sludge containing the re-usable abrasive grains is much smaller than the volume of the overflow. The classification effect results from the differences in settling velocities of the particles in the centrifugal field.
Larger particles sink faster, and they are favored to leave the centrifuge with the sludge. Smaller particles that settle with lower speed are easier carried and swept along with the liquid overflow of the centrifuge. The overflow is clarified by means of a second centrifuge resulting in a sludge to be discarded (small waste particles or fines in a small amount of lubricant fluid) and a more or less clarified stream of lubricant. Although this principle necessitates less operations than the first one described above and although it might be easier installed as a point-of-use process, two major disadvantages prevent the method from being widely used.
First of all, the second centrifuge is not capable of clarifying the overflow fraction of the first centrifuge satisfactorily to provide a readily re-usable lubricant. A substantial part of the fines remains in the lubricant and pollutes, when applied to prepare fresh slurry, this slurry from the beginning.
Secondly, the classification effect in the first centrifuge is far from being ideal: Substantial amounts of fines leave the centrifuge with the unspent abrasive particles contaminating fresh slurries prepared with the recovered abrasives in addition and limit the life time of the fresh slurries.
The reasons for this are some limitations of the centrifugal process:    i) High solids concentration of a common saw sludge favor particle-particle interactions; large particles can entrain fines along with them, forcing them to leave the centrifuge the wrong way out. The effect is strong especially near the inner wall of the centrifuge bowl where most of the solids are concentrated.    ii) The high viscosity of the lubricant amplifies the momentum transfer between the particles; small particles are much influenced by coarse ones.
The object of the present invention is to provide a method for treating spent abrasive slurries in a more economical way. Especially, the present invention provides a method for treating the spent abrasive slurry which may be carried out at the point of use obviating long transportation of the spent slurry for regeneration.